The invention relates to an electrical component which includes a circuit element, such as a solid electrolytic capacitor, arranged within a metallic casing or can. One electrical lead of the component is connected to the casing; the other lead is connected to the circuit element and extends through an opening in the casing. This opening is filled with a plastic material to seal the circuit element within the casing against environmental conditions.
Solid electrolytic capacitors consist of a sintered body constructed from a high purity metallic powder which is pressed around a corresponding metallic wire to form a cylindrical anode. Any one of a number of metals such as tantalum, niobium, aluminum or zirconium may be used, but tantalum is preferred. The anode is sintered at high temperatures in a vacuum into a porous body with a large metallic surface area. A metal oxide, such as tantalum pentoxide, is then formed on the internal and external surfaces by electrolysis to serve as the dielectric. Thereafter, a solid semiconductive oxide, such as manganese dioxide or lead oxide, is formed over the dielectric, for example in a pyrolysis oven conversion, to serve as the cathode. The structure is then dipped in graphite and silver to provide a conductive interface for the cathode connection.
The manufacture of this type of solid electrolytic capacitors may then proceed in the manner described in the U.S. Pat. No. 4,155,156. As is disclosed there, the anode wire connected to the cylindrical anode body is then flattened or notched and bent at the notch to make room for attachment of a solderable anode input wire. This input wire is then welded or soldered, in overlapping configuration, to the anode wire and the end of the anode wire is broken off.
These types of capacitors may be sealed in a number of ways. The simplest, most inexpensive procedure is to solder a second lead to the conductive coating of silver and to dip the entire device in hardenable low density plastic, such as epoxy resin. This type of seal is subject to environmental conditions, however, because the plastic is not impervious to moisture.
In a second, superior sealing process the capacitor is inserted in a solderable metal casing or can. The silver conductive coating is electrically and mechanically connected with the casing by embedding the capacitor in soft solder. A connecting wire is attached to the bottom of the casing to serve as the external cathode lead.
The anode wire with its attached input lead extends through an opening at the opposite end of the casing. This opening can be hermetically sealed by providing a metal-glass-metal cover. Both the metal anode wire and the metal casing are soldered to the respective metal portions of the cover, with the glass providing insulation between the anode wire and the casing.
In a third sealing process, the expensive metal-glass-metal cover is replaced by a hardenable plastic which fills the open end of the casing. Although this type of seal is less expensive, the plastic itself is pervious, particularly at the interface between metal and plastic: namely, along the anode wire lead and along the casing wall. Moisture may reach the inside of the casing via this interface, causing failure of the capacitor.